The invention embodied in the subject matter described herein was made during work financed by the following government contract: Department of Energy Office of Industrial Technologies Contract #DE-FC07-98ID13662.
This invention relates to aluminum electrolytic smelting cells and more particularly, it relates to collection and removal of molten aluminum from low temperature electrolytic cells for producing aluminum from alumina.
The use of low temperature (less than about 900xc2x0 C.) electrolytic cells for producing aluminum from alumina have great appeal because they are less corrosive to cermet or metal anodes and other materials comprising the cell. The Hall-Heroult process, by comparison, operates at temperatures of about 950xc2x0 C. This results in higher alumina solubility but also results in greater corrosion problems. Also, in the Hall-Heroult process, the carbon anodes are consumed during the process and must be replaced on a regular basis. In the low temperature cells, non-consumable anodes are used and such anodes evolve oxygen instead of carbon dioxide which is produced by the carbon anodes.
Non-consumable anodes are described in U.S. Pat. No. 5,284,562, incorporated herein by reference. That is, U.S. Pat. No. 5,284,562 discloses an oxidation resistant, non-consumable anode for use in the electrolytic reduction of alumina to aluminum, the anode having a composition comprising copper, nickel and iron. The anode is part of an electrolytic reduction cell comprising a vessel having an interior lined with metal which has the same composition as the anode. The electrolyte is preferably composed of a eutectic of AlF3 and either (a) NaF or (b) primarily NaF with some of the NaF replaced by an equivalent molar amount of KF or KF and LiF.
Other compositions for inert anodes are described in U.S. Pat. Nos. 4,399,008; 4,529,494; 4,620,905; 4,871,438; 4,999,097; 5,006,209; 5,069,771 and 5,415,742.
In U.S. Pat. No. 5,006,209, it is disclosed that finely divided particles of alumina are electrolytically reduced to aluminum in an electrolytic reduction vessel having a plurality of vertically disposed, non-consumable anodes and a plurality of vertically disposed, dimensionally stable cathodes in closely spaced, alternating arrangement with the anodes. A horizontally disposed gas bubble generator is located at the vessel bottom, underlying the cathodes and the spaces between each pair of adjacent electrodes. The vessel contains a molten electrolyte bath composed of (1) NaF+AlF3 eutectic, (2) KF+AlF3 eutectic and mixtures thereof, and in some cases (3) LiF. The alumina particles are maintained in suspension in the molten electrolyte bath by rising gas bubbles generated at the anodes and at the gas bubble generator, anodic liner, or anodic liner during the reduction process. However, having an anode located as the cell bottom is not without problems. In such cell, molten aluminum contacting the bottom anode becomes oxidized to aluminum oxide, interfering with the efficiency of the cell.
It will be appreciated that the low temperature cells have a lower solubility of alumina. Thus, excess alumina is provided in the electrolyte to insure a ready source of alumina. U.S. Pat. No. 5,006,209 discloses the use of gas bubbles generated at the anode to maintain the excess alumina particles in suspension. Thus, it will be seen that there is still a need to provide a bottom anode to produce gas bubbles. However, the use of a bottom anode interferes with collecting or removing aluminum produced in the cell.
There have been many different approaches to removing aluminum from an electrolytic cell. For example, U.S. Pat. No. 3,578,580 discloses a multicell furnace in which are mounted two bipolar electrodes 16, each of which is composed of an oxygen-ion conducting layer 17, a porous anode 18, the porosity of which is represented by a duct 19, and a cathode 20. The cathode consists for example of graphite or amorphous carbon in the form of calcined blocks or of some other electron conducting material which is resistant to the fused melt, such as titanium carbide, zirconium carbide, tantalum carbide or niobium carbide. The aluminum is separated at the cathodes and drops into collecting channels 21.
U.S. Pat. No. 4,795,540 discloses an electrolytic reduction cell for the production of aluminum having a slotted cathode collector bar. The slots are filled with insulating material thereby directing the electrical current flow through the cathode collector bar in a manner which reduces the horizontal current components in the cell.
U.S. Pat. No. 3,499,831 discloses a current collector pin adapted to be electrically connected to a graphite cathode block in an electrolytic cell, such as an alumina reduction cell, by insertion into a socket in the block, comprising a tubular copper conductive member surrounding and in contact with a central reinforcing metal core extending therethrough, and an outer sleeve surrounding and extending over the portion of the length of the tubular member not inserted into the socket.
U.S. Pat. No. 4,194,959 discloses an electrolytic reduction cell for the production of aluminum having current collector bars running across the floor of the cell unitarily or in separate sections. Deformation of the molten metal/electrolytic bath interface is reduced by leading current out of the collector bars or bar sections at positions remote from their ends by connector bars connected to said positions.
U.S. Pat. No. 4,392,925 discloses that the durability of oxide-ceramic anodes can be increased, if the aluminum surface which lies opposite the active anode surface and is in direct contact with the molten electrolyte, is smaller than the active anode surface. The separated aluminum is collected on the floor of the carbon lining and is subdivided by an insulating material into pools, which are connected together by means of tubes or channels. The total of all the aluminum surfaces exposed to the melt amounts to 10-90% of the active anode surface. Further, it is noted that aluminum produced during electrolysis flows along the cathode as a film and is collected in an aluminum pool 38, arranged on the floor of the cell which communicates via pipes with an aluminum collection tank.
It is an object of this invention to provide a method for removing molten aluminum from an electrolytic cell used for producing aluminum from alumina.
It is another object of this invention to provide a method for removing aluminum from a low temperature cell for producing aluminum from alumina.
It is still another object of the invention to provide a process and apparatus for removing aluminum from an electrolytic cell employing a gas bubble generator or bottom anode for generating gas bubbles during operation of the electrolytic cell for producing aluminum from alumina.
It is yet another object of this invention to provide a process for removing aluminum from a low temperature electrolytic cell employing a bottom anode while avoiding contacting said anode with aluminum.
These and other objects will become apparent from a reading of the specification and claims appended hereto.
In accordance with these objects, there is provided a method of producing aluminum in an electrolytic cell containing alumina dissolved in an electrolyte, the method comprising the steps of providing a molten salt electrolyte having alumina dissolved therein in an electrolytic cell containing the electrolyte. A plurality of nonconsumable anodes and a plurality of cathodes are disposed in the electrolyte. The cathodes are connected to the bus bar outside the cell using a connection means comprising a flexible metal strap having a first end electrically connected to the bus bar, and a collector bar comprised of an electrically conductive, dimensionally stable material resistant to attack by electrolyte and by molten aluminum. The collector bar has a first end having a metal cap cast thereon to provide electrical contact with the collector bar. The flexible strap has a second end electrically connected to the metal cap, the collector bar having a second end electrically connected to the cathode underneath the surface of the electrolyte. An electric current is passed through the anodes and through the electrolyte to the cathodes to deposit aluminum on the cathodes and molten aluminum is collected from the cathodes.